Direct-viewing color storage tube



Oct. 21, 1958 s, HANSEN DIRECT-VIEWING COLOR STORAGE TUBE 2 Sheets-Sheet1 Filed July 16. 1953 BAIA Oct. 2l, 1958 s, HANSEN I 2,857,551

DIRECT-VIEWING COLOR STORAGE TUBE United States Patent O DIRECT-VIEWINGCOLOR STORAGE TUBE Siegfried Hansen, Los Angeles, Calif., assignor, bymesne assignments, to Hughes Aircraft Company, a corporation of DelawareApplication July 16, 1953, Serial No. 368,341

5 Claims. (Cl. 315-12) This invention relates to a direct-viewing colorstorage tube and more particularly to a storage tube capable ofsimultaneously producing a visual presentation of several multi-coloredimages.

One specific embodiment of the tube of the present invention isparticularly adaptable to search radar systems. This embodiment providesfor the presentation of two colors such as, for example, red and green.As is generally known, a type B search display presents the range andazimuth of a target. The tube of the present invention would enable atarget indication to be presented in either red or green, depending uponits relative elevation, when incorporated in a radar system of thistype. Alternatively, the tube would enable certain types of targets tobe presented n color for identification purposes. Another embodiment ofthe directviewing color storage tube capable of presenting three coloredimages simultaneously, is, of course, particularly adaptable to colortelevision systems.

The device of the present invention incorporates color presentationtechniques in a direct-viewing storage tube of the type disclosed in acopending United States Patent application by Siegfried Hansen, SerialNo. 299,363, led July 17, 1952, now Patent 2,788,466, and entitled"Method and Apparatus for Utilizing Electron Bombardment Induced SurfaceConductivity by the same applicant. The tube of the present inventioncomprises an electron writing gun for each colored image to bepresented. The electron beams produced by these writing guns are causedto scan rasters in register with each other on a storage surfaceprovided by a foraminous target element. A mask is disposed adjacent tothe target element and interposed between the electron beams and thestorage surface. This mask is such that each electron beam is incidenton non-overlapping areas of the storage surface. Each electron beam isintensity modulated in accordance with its respective colored image toproduce a distinct charge pattern representative of each colored imageon the scanned areas of the storage surface.

A viewing screen, disposed adjacent to the forarninous target element onthe side opposite from the electron guns, has appropriate coloredphosphor areas disposed in register with the areas of each chargepattern of correspondingl color. A simultaneous visual presentation ofthe several charge patterns is then produced by directing floodelectrons through the foramina of the target element contained withineach elemental area of the storage surface in proportion to the chargethereon in a collimated beam to the viewing screen.

A basic problem that is generally encountered in colored televisionimage presentation systems is that of flicker. This problem stems fromthe limited bandwidth allocated for the transmission of the frequenciesconstituting the image signal. This limited bandwidth requires that acompromise be made between the number of complete images or framesproduced each second and the definition of the images. The number offrames 2,857,551 Patented Oct. 21, 1958 ICC produced each second cannotbe reduced beyond the point at which icker occurs without concomitantundesirable eiects. Hence the bandwidth available for deiniton purposesis determined by the frame repetition rate.

In the case of a color presentation, three distinct colored imagesgenerally constitute one frame. It is necessary that the repetition ratefor each colored image be high enough to prevent flicker when viewedalone, as it is always possible for one color to predominate throughoutlarge portions of the presentation. Thus, it is seen that a colorpresentation having the same denition as a black and white presentationwould presumably require three times the bandwidth. In this connection,reference is made to an article entitled Alternative Approaches to ColorTelevision" by Donold G. Fink, which appears on pages 1124 to 1134 invol. 39 of the Proceedings of the I. R. E., October 1951, where theabove problem is discussed at length.

The advantages of the direct-viewing color storage tube of the presentinvention include, of course, all the advantages attributable to thedirect-viewing type storage tube described in the aforementionedcopending application by the same inventor, Siegfried Hansen. The moreimportant advantages of this tube are the continuous bright display ofthe charge pattern, the controllable effective persistence of thescreen, and the fast writing speed accomplished by the use ofbombardment induced conductivity techniques. In its adaption to colorpresentation, it is quite obvious that the effective persistence of eachimage may be controlled so that the several different coloredpresentations may be made to appear simultaneously, thereby considerablydecreasing the undesirable llicker. Employment of the color tubedisclosed herein would enable the frame rate of successive presentationsto be decreased to the extent necessary to provide for continuity ofmotion, thus creating a substantial saving in bandwidth which may beutilized to improve definition.

It is therefore an object of this invention to provide a direct-viewingcolor storage tube.

Another object of this invention is to provide a direct-viewing storagetube capable of simultaneously producing a plurality of multi-coloredpresentations of controlled persistence.

Still another object of this invention is to provide a direct-viewingcolor storage tube incorporating a separate writing beam for each colorand an appropriate mask disposed in front of a storage surface scannedby the writing beams in such a manner that each writing beam is incidenton distinct non-overlapping areas of the storage surface.

A further object of this invention is to provide a direct-viewing colorstorage tube utilizing flood electrons to simultaneously produce avisual presentation of a plurality of multi-colored images.

The novel features which are believed to be characistie of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawings, in which an embodiment of the invention isillustrated by way of example. It is to be expressly understood that thedrawings are for the purposes of illustration and description only, andare not intended as a definition of the limits of the invention.

Fig. 1 is a perspective view of a two-color embodiment of the tube ofthe present invention;

Fig. 2 is an enlarged cross sectional view of the tube shown in Fig. 1taken along line 2 2 of that figure;

Fig. 3 is a perspective View of an enlarged portion of the color mask;

Fig. 4 is a perspective View of an enlarged portion of the targetelement; and

Fig. 5 is a cross sectional view of the geometrical relationship betweenthe electron beams and enlarged portions of the color mask, targetelement, and viewing screen.

Referring now to Fig. 1, there is shown a perwective view of a tube ofthe present inventionadapted for twocolor presentation which isparticularly suitable for radar applications. This tube comprises anevacuated envelope having neck portions 12, 14, 16 and 18 disposed asshown in the ligure. Neck portions 12, 14 house electron guns 20 and 22,respectively, shown in schematic form, for producing electron writingbeams. Electron beam deilecting means 24 and 26 Vare disposed axiallyabout the electron beams produced by guns 20 and 22, respectively.Defiecting means 24 and 26 are energized by appropriate beam deectioncontrol signals to cause the electron beams to trace identical rasterson a target element 30 through a color mask 28.

Neck portions 16, 18 are disposed immediately above the neck portions12, 14, and house sources of flood electrons 32, 34, respectively. Thesources of iiood electrons are arranged so that iiood electrons from therespective sources 32, 34 are directed uniformly over the area of targetelement 30 through color mask 28. The ilood electrons are directedthrough each elemental area of color mask 28 at an angle of incidencesubstantially the same as the angle of incidence of the electron writingbeams when scanning the same elemental area. This is accomplished bypositioning the sources 32, 34 of ood electrons as near to therespective paths of the electron writing beams as reasonably possible.

In order to explain more clearly the structure and manner of operationof the tubel of the present invention, reference is made to Fig. 2. Ascan be seen in this figure, neck portions 12, 14 are arranged at anacute angle with each other so that the electron writing beams, producedby electron guns 20, 22 housed therein, are incident on target element30 at substantially different angles over the entire area of itssurface. Target element 30 is disposed in the right portion of envelopelt) facing the electron writing beams as shown in Fig. 2. Color mask 28is positioned adjacent to target element 30 so as to periodicallyintercept the electron beams when scanning target element 30. A viewingscreen 36 is disposed adjacent to and in register with target element 30on the side opposite from the color mask 28. An electrode 38 provides adrift region between deccting means 24, 26 and the color mask 28.

Color mask 28 comprises a glass core 40 having a thin coating 41 ofmetal such as, for example, aluminum disposed uniformly over its outersurface. The glass core 40 may be fabricated by employing the CorningPhotoform Process" and the thin coating 41 of aluminum applied thereonby evaporation. A small portion of mask 28, as employed in the two-colorembodiment of the tube of the present invention, is shown in perspectivein Fig. 3. The openings 42 for the two-color embodiment are equal inwidth to the solid sections and are several times their width in length.For example, the solid sections and the width of the openings may be ofthe order of 0.01" and the length of the openings may be of the order of0.02". Cross pieces 43 which separate the openings 42 are for structuralpurposes and serve no direct function in the operation of the tube. Themask 28 is scanned by the electron beams in a direction substantiallyperpendicular to the length of the openings. The solid portions of themask not exposed to the electron beams are made substantially narrowerthan the exposed solid portions so as not to intercept electronsentering openings 42 which will generally be penetrating through themask 28 at an angle.

Target element 30 is shown in cross section in Fig. 2,

and an enlarged portion of it is illustrated in Fig. 4. Target element30 comprises a stainless steel wire screen or electroformed nickelscreen 45 having of the order of 300 wires per inch; screen 45 serves asa contrast control grid 45. A layer 46 of dielectric material such as,for example, talc, is disposed over grid 45, and an aluminum mesh 47,having of the order of meshes per inch, is in contact with layer 46 toserve as a collector grid 47. The dielectric material comprising layer46 must exhibit both bombardment induced conductivity and secondaryelectron emission characteristics. The surface of layer 46 appearingbetween the openings of collector grid 47 provides a storage surface 48.

Color mask 28, electrode 38, and collector grid 47 of target element 30are al1 maintained at a potential of the order of 200 volts positivewith respect to ground by means of connections to the positive terminalof a potential source 49 and contrast control grid 45 is maintained at apotential of the order of -10 volts with respect to ground by means of aconnection to the negative terminal of source 49, an appropriateintermediate tap of which is connected to ground.

Viewing screen 36, appearing in cross section in Fig. 2, comprises aglass pane 50 which may be the tlat portion on the extremity of envelope10 as viewed in the figure, a transparent conductive coating 51 and aphosphor layer 52. Transparent conductive coating 51 is maintained at apositive potential of the order of 5000 volts with respect to ground bymeans of a connection to the positive terminal of a potential source 54,the negative terminal of which is connected to ground.

Phosphor layer 52 consists of a series in parallel, horizontal stripesof color phosphor having essentially the same vertical dimension as thevertical dimensions of the openings in color mask 28. These stripes maybe alternately of, for example, red and green phosphors. The pitch ofthese stripes must have a pitch slightly greater than that of theopenings of the color mask to allow for the parallax of the electronbeam as it passes from mask 28 to the storage surface 48 of targetelectrode 30. The manner in which such a screen having alternate colorltripes with parallax correction may be produced is described in U. S.Patent No. 2,568,448 entitled Parallax Correction in Color Televisionwhich issued September 18, 1951, to this inventor, S. Hansen.

In order to more adequately illustrate the relation of the color stripeson the viewing screen 36 to the openings in the color mask 28, referenceis made to Fig. 5. This figure illustrates enlarged portions of thecolor mask 28, target element 30 and viewing screen 36 disposed incorrect relationship with one another to produce a two-colorpresentation with two electron beams having centers of deection atpoints 56 and 58. Lines 57, 59 connect the extremities of the openingsof color mask 28 with the centers of deection 56, 58 of the electronbeams, respectively, each beam being modulated in accordance with adifferent color image such as, for example, red and green. The dashedareas between lines 57, 59 indicate the regions where the electron beamswill penetrate through color mask 28 to target element 30.

Proper operation of the tube of the present invention requires that theareas of storage surface 48 scanned by one electron beam do not overlapwith the areas scanned by the other beam. Each beam will produce acharge on the scanned areas of the storage surface 48, in a mannerhereinafter described in more detail, to produce two integrated chargepatterns.

Another phase of the operation of the tube involves directing oodelectrons through the interstices included in each elemental area ofstorage surface 48 in proportion to the charge thereon to the viewingscreen 36. The red stripes of phosphor layer 52, designated by Rf aredisposed so as to be in register with the areas of storage surface 48scanned by the electron beam modulated in accordance with the red image.Similarly, the green stripes, designated by G are disposed so as to bein register with the areas of storage surface 48 scanned by the electronbeam modulated in accordance with the green image. Thus, as illustratedin Fig. 5, it is seen that the pitch of the color stripes comprisingphosphor layer S2 must be somewhat greater than the pitch of theopenings in color mask 28. The exact pitch, of course, is a function ofthe distances of the centers of deection 56, 58 from each other and fromthe storage surface 48 of target element 30, the distance between colormask 28 and target element 30, and the size of the openings in colormask 28.

The operation of the tube of the present invention may be divided intothree phases, namely, the producing of the charge replicas of the colorimages on the storage surface 48, the simultaneous production of avisual presentation of the color images, and the returning of thepotentials constituting the charge replicas to a substantially tixedreference potential such as, for example, ground, prior to the producingof a subsequent charge replica on the storage surface 48. During theactual operation of the tube, these three phases of operation aregenerally taking place at the same time.

During the first phase of operation, the electron beams produced byelectron guns 20, 22 (see Fig. 2) are caused to scan identical rasterson target element 30 through color mask 28. Alternate segments of thelines of each raster are intercepted by the color mask 28 so that thetwo electron beams together scan a single integrated raster on storagesurface 48. As explained in the third phase of operation of the tube,the Hood electrons charge the storage surface 48 of target element 30 tosubstantially ground potential. Hence a positive potential gradientexists from the storage surface 48 to the collector grid 47 inasmuch asthe collector 47 is maintained at a potential of the order o-f 200 voltspositive with respect to ground.

The cathodes of electron guns 20, 22 are maintained at a potential ofthe order of 3000 volts negative with respect to ground and therespective electron beams intensity modulated with the color imagesignals. An electron beam, upon being scanned over an elemental area ofstorage surface 48, releases electrons from their molecular bonds withinthe surface layer of dielectric material 46 in proportion to theintensity of the electron beam thereon. The electrons released fromtheir molecular bonds may be referred to as conduction electrons. Theseconduction electrons are attracted to collector grid 47 through themolecular matrix comprising the surface layer of the dielectric material46 to effect the charging of the scanned elemental area of storagersurface 48 in a positive direction. Two integrated nonoverlappingcharge replicas of the color images are thus produced on the storagesurface 48 since the number of conduction electrons produced Within eachelemental area thereof is proportional to the'intensity of the electronbeam at the time of scanning the elemental area.

The second phase of operation comprises the directing of flood electronsthrough the interstices contained within each elemental area of thestorage surface 48 in a collimated beam to the viewing screen 36 toproduce a simultaneous presentation of the color images. The rst step ofthis phase of the operation is the directing of the flood electronsuniformly over the area of the storage surface 48 which is complicateddue to the color mask 28 being disposed between the sources of the floodelectrons and the target element 30. This complication is overcome bydisposing the tiood guns 32, 34 in the proximity of the centers ofdeection of the electron writing `beams produced by electron guns 20,22, respectively. Thus the ilood electrons penetrate through the colormask 28 at substantially the same angle as the associated electronwriting beam and, hence, are directed over substantially the same areasscanned by the electron beam.

The next step in the second phase of operation is to direct the floodelectrons through the interstices within each elemental area of thestorage surface 48 in proportion to the charge predominating thereon ina collimated beam to the viewing screen 36. In order to accomplish this,the cathodes of the flood guns 32, 34 are maintained at substantiallyground potential so that, in the absence of a positive charge on storagesurface 48, the flood electrons are repelled by negative voltage appliedto contrast control grid 45. The effect of a positive charge establishedon storage surface 48 is to combine with the electric eld from viewingscreen 36 to produce field penetration through the contrast control grid45. The increased predominance of a positive charge on an elemental areaof storage surface 48, of course, produces increased field penetrationthrough contrast Vcontrol grid 45 to the viewing screen 36, thusallowing increased numbers of Hood electrons to flow through theinterstices within the elmental area of storage surface 48. Theelectrons penetrating through target element 30 are accelerated in acollimated beam to the appropriate color stripes of phosphor layer 50 bythe high positive voltage applied to conductive coating S1 to produce asimultaneous visual presentation of the charge replicas of the colorimages.

The third phase of operation of the tube of the present inventioncomprises returning the potentials constituting the charge replica to areference potential prior to being charged positive again by an electronwriting beam. This phase of the operation is controlled by the potentialof the collector grid 47 relative to the potential of the cathodes offlood guns 32, 34. As previously stated, it was specified that the layer46 of dielectric material which provides storage surface 48 possessessecondary electron emission characteristics. Thus storage surface 48will have a concomitant critical potential where the secondary electronemission ratio is unity. Maintaining the collector grid 47 atapproximately 1.3 times this critical potential with respect to thecathodes of iood guns 32, 34 will produce normal persistence of thecharge replicas. The function of the process of returning the potentialsconstituting the charge replica to a reference potential consists firstin charging all the potentials less than the critical potential to thepotential of the cathodes of the flood guns 32, 34 simultaneously withthe charging of the potentials greater than the critical potential tothe potential of the collector grid 47 by secondary electron emission.In general, the entire area of storage surface within an opening of grid47 will be either charged to the potential of the cathodes of the iloodguns or to the potential of the collector grid 47. Continued action ofthe Hood electrons on an area charged to the potential of the collectorgrid 47 causes this area to shrink away from the meshes of grid 47 untilthe entire area is at the potential of the flood gun cathode.

It is realized that other modes of operation may be effected by merelyincreasing or decreasing the voltage of collector grid 47 relative tothe critical potential. That is, increasing this voltage will increasethe persistence by decreasing the rate at which the areas of storagesurface within the openings of grid 47 that are charged to the potentialthereof will shrink until the charged areas re- ,main substantiallyfixed in size. Utilization of this mode of operation would requireadditional means for erasing the charge replica prior to producing asubsequent one. On the other hand, the collector grid voltage may belowered until the entire initial charge produced by the writing beams isdischarged directly to the potential of the flood gun cathodes. Thislatter mode may be desirable for television applications of thedisclosed tube, whereas the former mode is adaptable to certain types ofradar presentations.

For television applications it would be necessary to use at least threedifferent electron guns for writing purposes. In the event that it isdesired to use the same type of mask as mask 28 of the embodimenthereinbefore described, it would be necessary to reduce the size of theopenings 42 to approximately one-haii?V the width of' the solidsections. The electron guns would be angularly disposed with each otherin a common plane in order that the electron beams produced by the gunspenetrate through the openings of the mask at diiferent angles. Aviewing screen having phosphor stripes of an appropriate color inregister with the area of the storage surface scanned by each electronbeam and 'flood guns disposed near the center of deection of eachelectron beam are employed -in the same manner as in the priorembodiments.

It is, of course, apparent that various other modifications of themasking arrangements may be employed in practicing the disclosedinvention which are within the teachings of the present disclosure.

What is claimed as new is:

l. A direct-viewing electronic color storage tube for the visualpresentation of a plurality of image signals in color, said tubecomprising means for producing a plurality of electron beams, means forcontrolling the intensity of each of said electron beams with an imagesignal, a storage screen having a storage surface on one side thereof,means for scanning intermittent non-overlapping elemental areas of saidstorage surface with each of said electron beams, said areas for eachbeam being intermittent in the direction of scan and interlaced in thedirection of scan with the intermittent areas scanned by the other saidbeams to produce a plurality of integrated charge replicas on saidstorage surface corresponding, re-

spectively, to said image signals, a viewing screen disposed coextensivewith and adjacent to said storage screen on the other side thereof, saidviewingI screen including a duoresceni screen for'developinglight oi'predetermined color in register with each of said plurality of chargereplicas, and means for directing. a collimated flow of electronsthrough said storage-screen in proportion to the charge on saidstorage-surface to produce a color presentation of each of said chargereplicas on said viewing screen.

2. The direct-viewing electronic color storage tube as defined in claiml wherein said means for scanning intermittent non-overlapping elementalareas of said storage screen with each of said electron beams includes aconductive mask disposed adiacent to said storage screen on said oneside thereof to periodically intercept said electron beams.

3. A direct-viewing electronic color storage tube for the visualpresentation of a plurality of image signals in color, said tubeAcomprising means for producing a plurality of high energy electronbeams; means for controlling the intensi-ty of each of said electronbeams with an image signal; a target element including a conductivescreen, a layer of dielectric material that exhibits secondary electronemission disposed on one side of saidy conductive screen coextensivewith the meshes thereof, and a conductive mesh disposed in contact withsaid layer o`i dielectric material whereby the portions of the surfaceof said layer of dielectric material within the openings of said meshprovide a storage surface; means for maintaining said conductive mesh ata i'i-rst potential level; means for maintaining said storage surf-aceat a second potential level negative with respect to said" firstpotential level to produce a positive potential gradient from saidstorage surface to said conductive mesh; means for scanning intermittent non-overlapping elemental areas of said target element witheach of said electron beams, said areas for each beam being intermittentin the direction of scan and interlaced in the direction of scan withthe intermittent arcas scanned by the other said beams to liberatesecondary electrons from said storage surface in proportion to the beamintensity thereon whereby saidy secondary electrons are attracted tosaid conductive mesh by said potential gradient to produce a pluralityof integrated charge replicas on said storage surface corresponding,respectively, to said image' signals; a viewing screen disposedcoextensive with and adjacent to said conductive screen on the otherside thereof, said viewing screen including a fluorescent screen adaptedto provide light of predetermined color in register with each of saidplurality of integrated charge replicas; and means for directing acollimatd ow of electrons through said 'target element in proportion tothe charge thereon to said viewing screen to produce a colorpresentation of each of said charge replicas.

4. A direct-viewing electronic color storage tube for the visualpresentation of two image signals in color, said tube comprising firstand second electron guns for producing, respectively, first and secondhigh energy electron bea-ms of elemental cross sectional area; means forcontrolling the intensity of said first and second electron beams withthe image signals; a target element including a conductive screen, alayer of dielectric material that exhibits secondary electron emissiondisposed on one side of said conductive screen coextensive with theconductors thereof, and a conductive mesh disposed in contact with saidlayer of dielectric material, to separate the surface of said layer ofdielectric material within the openings of said mesh to provid-e asubdivided storage surface; means for maintaining said conductive meshat a first potential level; means for maintaining said storage surfaceat a second potential level negative with respect to said firstpotential level to produce a positive potential gradient from saidstorage surface to said conductive mesh; means for scanning alternatenon-overlapping elemental areas of said target element with said firstand second electron beams to liberate secondary electrons from eachscanned elemental area of said storage surface in proportion to the beamintensity thereon, said secondary electrons being attracted to saidconductive mesh by said potential gradient to produce two sets ofintegrated charge replicas on said storage surface corresponding to saidtwo image signals; a viewing screen disposed coextensive with andadjacent to said conductive screen on the other side thereof, saidviewing screen including a iluorescent screen for developing light ofpredetermined color in register with each of said two integrated chargereplicas; and means including an electron flood gun disposed adjacentthe path of each of said electron beams for directing respectively acollimated ilow of electrons through said target element in proportionto the two sets of charge replicas thereon to said viewing screen toproduce a color presentation of said charge replicas.

5. A direct-viewing electronic color storage tube for the visualpresentation of a plurality of image signals in color, said tubecomprising means for producing a plurality of high energy electronbeams; means for controlling the intensity of each of said electronbeams with an image signal; a target element including a conductivescreen, a layer of dielectric material that exhibits secondary electronemission disposed on one side of said conductive screen coextensive withthe conductors thereof, and a conductive mesh disposed in contact withsaid layer of dielectric material to separate said target element intoportions of the surface of said layer of dielectric material within theopenings of said mesh to provide a subdivided storage surface; means formaintaining said conductive mesh at a first potential level; aconductive mask disposed coextensive with an adjacent to the storagesurface of said target element; means for directing ood electronsemanating from a second potential level that is negative with respect tosaid rst potential level through said mask and uniformly over saidtarget element to charge each elemental area of storage surfacesubstantially to said second potential level to produce a potentialgradient from said storage surface to said conductive mesh; means forscanning each of said beams over said target element through said maskfor bombarding intermittent non-overlapping areas of said storagesurface by each of said electron beams to liberate secondary electronsfrom said storage surface in proportion to the intensity of the electronbeam thereon which are attracted to said conductive 10 mesh by saidpotential gradient to produce a plurality of References Cited in the leof this patent integrated charge replicas on said storage surfacecorresponding, respectively, to said image signals; a viewing UNITEDSTATES PATENTS screen disposed contiguous to and coextensive with said2,532,339 Sehlesinger Dec. 5, 1950 taetlelmenhavigg a dferent Golgi'lhosPtlof fff-eh 5 2,547,638 Gardner Apr. 3, 1951 sai e ec ron eam 1nregis er respec ve y wi sai 1n ermittent non-overlapping areas of saidstorage surface, 2659026 Eps'tem Nov' 10 1953 whereby said Hoodelectrons penetrate through the target 674504 Welmel' APr- 6 1954element in proportion to the charge on said storage sur- 2,761,089 HaeffAug. 28, 1956 face to produce a color presentation of each of said 10charge replicas on said viewing screen.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No.2,857,551 October 2l, 1958 Siegfried Hansen It is hereby certified thaterror appears in the printed specification of the above numbered patentrequiring correction and that Jche said Letters Patent should read ascorrected below.

Column l, lines 39 and ,O, for "Method and Apparatus for UtlizngElectron Bomberdment Induced Surface Conductivty` read Direct-Viewing 8,line 62, for with an" read with and Signed and sealed this 3rd day of'March 1959.

(SEAL) Commissioner of Patents Attesting Ocer

